It is known that in the operation of pump-turbines, in a turbine mode, the pump-turbines are started, in the manner of any reaction turbine, by simultaneously opening the wicket gates. This causes the pump-turbine runner to accelerate from a stationary position to rated revolutions per minute. When the wicket gates are positioned at an idle running position for obtaining an idle running operation of the pump-turbine, the motor-generator can be synchronized and connected to the associated electric grid system. Upon further movement of the wicket gates toward an open position, the pump-turbine will operate as a turbine and will furnish power. The characteristic curve of pump-turbines become steeper and steeper with an increasing n.sub.1 in the n.sub.1 -Q.sub.1 diagram (unit number of revolutions over unit flow). The characteristic curves may even become vertical or have a negative direction. This means that the characteristic performance of the pump-turbine in this range may be erratic.
A cutout of the four quadrant characteristic curve diagram of a given pump-turbine is shown in FIG. 1. As there shown, the curves for various wicket gate openings are identified as a.sub.1 through a.sub.5 where the largest wicket gate opening a.sub.5 is used for synchronization. The L-curve in FIG. 1 is the idle running operation curve for turbine operation with a movement of M=O. Below idle running curve L in area B, FIG. 1 depicts the so-called braking operation of a pump-turbine and indicates that the runner keeps absorbing power and that the flow is in turbine operation direction and is operating as a brake-up to the point when the flow of the turbine becomes zero. Further, below the abscissa the pump-turbine enters into a pumping operation in turbine rotation direction, i.e., it enters in what is called a reverse pumping operation. The water will be delivered in the pump flow direction in spite of the fact that the pump-turbine is running in a turbine rotational direction.
In braking and in reverse turbine operation the curves, as shown in FIG. 1, depict an S-shape configuration; this condition causes undesirable effects such as the instability of operation of the pump-turbine. The result of this type of instability of operation is that synchronization in turbine mode operation is extremely difficult. Also, running idle for an extended period of time is impossible. The reason for this is that the smallest unavoidable pressure variations may cause sudden irregularities at a point when a load is taken on. In the transition period from synchronous operation in the direction of rotation in turbine mode, the following takes place: (a) the runner operates in air in preparation to operation in the turbine mode; (b) when the turbine is filled with water and the valve is opened, the temporary increase in power becomes undesirably large when the wicket gates are opened; (c) upon a further increase in the openings between the wicket gates a suddenly large power surge will be experienced when the pump-turbine starts to operate as a turbine.